1. Field of the Invention
This invention relates to a system for feedback control of the air/fuel ratio in an internal combustion engine, in which an oxygen sensor element is disposed in the exhaust gas. The sensor element is of the solid electrolyte oxygen concentration cell type provided with an electric heater to ensure proper functioning of the concentration cell and is operated with a supply of a DC current to maintain a reference oxygen partial pressure therein. More particularly, the present invention relates to a sub-system for controlling the supply of the current to the concentration cell in such an oxygen sensor element.
2. Discussion of the Prior Art
In recent internal combustion engines and particularly in automotive engines, it has become popular to perform electronic feedback control of air/fuel ratio by utilizing an oxygen sensor installed in an exhaust passage to provide an electrical feedback signal indicative of the air/fuel ratio of an air-fuel mixture actually supplied to the engine. Based on this feedback signal a control circuit commands a fuel-supplying apparatus such as electronically controlled fuel injection valves to regulate the rate of fuel feed to the engine so as to correct deviations of actual air/fuel ratio from an intended value.
Usually the aforementioned oxygen sensor is of the concentration cell type having a layer of an oxygen ion conductive solid electrolyte such as zirconia containing a small amount of a stabilizing oxide. In this field, a recent trend is to miniaturize the oxygen-sensitive element of the sensor by constructing it as a laminate of thin, film-like layers on a very small plate-shaped ceramic substrate. In an oxygen sensor element of this type it is necessary to maintain a reference partial pressure of oxygen at the interface between the solid electrolyte layer and a reference electrode layer in the laminate. As described in U.S. Pat. No. 4,224,113, a reference oxygen partial pressure of a nearly constant level can be maintained in this sensor element by continuously supplying a DC current on the order of 10.sup.-6 to 10.sup.-5 A to the concentration cell part of the sensor element. This current flows through the solid electrolyte layer thereby forcing oxygen ions to migrate in the solid electrolyte layer in a predetermined direction. Since the solid electrolyte does not function properly at temperatures below a certain level, such as about 400.degree. C., the substrate of the oxygen sensor element is provided with a heater to which an adequate voltage is applied to maintain the sensor element at a nearly constant temperature.
In practice there is some probability that the heater in the oxygen sensor element would break during long use of the sensor element in the exhaust gases due to frequent changes in both temperature and flow velocity respectively over wide ranges. When the heater breaks, the output of the oxygen sensor element begins to falsely indicate that the oxygen concentration in the exhaust gas remains at a considerably low level, or, in other words that the actual air/fuel ratio in the engine is constantly lower than the intended value, irrespective of the actual oxygen concentration in the exhaust gas. Therefore, the control circuit in the air/fuel ratio control system is designed to interrupt the feedback control of air/fuel ratio if the output of the oxygen sensor element continues to indicate that the actual air/fuel ratio is at a relatively low level for a predetermined length of time and, instead, to produce a constant control signal to keep the rate of fuel feed to the engine at a predetermined constant value corresponding to an air/fuel ratio value which is somewhat lower than the optimum air/fuel ratio determined by the feedback control. However, it is inevitable that during the monitoring period before the shift from the closed-loop control to the open-loop control, the rate of fuel feed is varied on the basis of the incorrect feedback signal provided by the oxygen sensor element suffering from the broken heater. If the heater breaks while the actual air/fuel ratio is above the target value, the closed-loop control of air/fuel ratio during the monitoring period results in a serious problem because the control circuit continues to put out a control signal that causes further increase in the air/fuel ratio in response to the incorrect feedback signal, so that the engine is fed with an excessively lean mixture. Consequently the engine is liable to lose operational stability and even stall in some cases.